The present invention relates to magnetic resonance imaging (MRI) radio frequency (RF) coils and, in particular, to a coil for imaging a human shoulder.
MRI utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field of the main magnet (named B0—the main magnetic field in MRI physics). The magnetically polarized nuclear spins generate magnetic moments in the human body. The magnetic moments point in the direction of the main magnetic field in a steady state, and produce no useful information if they are not disturbed by any excitation.
The generation of a nuclear magnetic resonance (NMR) signal for MRI data acquisition is accomplished by exciting the magnetic moments with a uniform RF magnetic field (called the B1 field or the excitation field). The B1 field is produced in the imaging region of interest by an RF transmit coil which is driven by a computer-controlled RF transmitter with a power amplifier. B0 and B1 refer to the magnetic axes of the fields. During excitation, the nuclear spin system absorbs magnetic energy, and it's magnetic moments process around the direction of the main magnetic field. After excitation, the processing magnetic moments will go through a process of free induction decay (FID), releasing their absorbed energy and returning to the steady state. During FID, NMR signals are detected by the use of a receive RF coil, which is placed in the vicinity of the excited volume of the human body. The NMR signal is the secondary electrical voltage (or current) in the receive RF coil that has been induced by the precessing magnetic moments of the human tissue. The receive RF coil can be either the transmit coil itself, or an independent receive-only RF coil. The NMR signal is used for producing magnetic resonance images by using additional pulsed magnetic gradient fields, which are generated by gradient coils integrated inside the main magnet system. The gradient fields are used to spatially encode the signals and selectively excite a specific volume of the human body. There are usually three sets of gradient coils in a standard MRI system, which generate magnetic fields in the same direction as the main magnetic field, varying linearly in the imaging volume.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In a standard MRI system, the best excitation field homogeneity is usually obtained by using a whole-body volume RF coil for transmission. The whole-body transmit coil is the largest RF coil in the system. A large coil, however, produces a lower signal-to-noise ratio (SNR) if it is also used for reception, mainly because of its greater distance from the signal-generating tissues being imaged. Since a high signal-to-noise ratio is the most desirable in MRI, special-purpose coils are used for reception to enhance the SNR ratio from the volume of interest.
In practice, a well-designed specialty RF coil should have the following functional properties: high SNR, good uniformity, sufficient coverage and penetration, a high unloaded quality factor (Q) of the resonance circuit, and a high ratio of the unloaded to loaded Q factors. In addition, the coil device should be mechanically designed to facilitate patient handling and comfort, and to provide a protective barrier between the patient and the RF electronics. Another way to increase the SNR is by quadrature phased array reception. In this method, NMR signals are detected in two orthogonal directions, which are in the transverse plane or perpendicular to the main magnetic field. The two signals are detected by two independent individual coils that cover the same volume of interest. With quadrature reception, the SNR can be increased by up to a factor of the square root of two over that of individual linear coils.
Imaging a human shoulder should include imaging and visualization of the shoulder girdle including glenoid fossa, labrum, humeral head, neck and body, supraspinatus, infraspinatus and teres minor insertion (rotator cuff) and surrounding soft tissues. Shoulder imaging comprises approximately 20% of all MRI done. Most of these examinations are done for the following reasons:                a). R/O rotator cuff tear (new or current)        b). R/O labral tear or deterioration        c). R/O impingement syndrome        
The radiographical applications associated with the evaluation of shoulder anatomy are conventional X-ray and X-ray anthrography, sometimes enhanced with contrast agents. MRI shoulder imaging has been used in place of X-ray anthrograms and combined X-ray and enhanced CT scans. The main reasons for MRI as a preferred modality is reduction of patient exposure to ionizing radiation and iodinated contrast agents. Therefore, MRI examinations of the shoulder are better for a patient in terms of morbidity and is more cost effective for the health provider.
Several problems exist in existing MRI shoulder coils. In a horizontal bore MRI scanners, the quadrature coils of U.S. Pat. Nos. 5,343,862 and 5,351,688) do not provide optimized SNR due to the required separation of the anterior and posterior elements. The separation between the anterior and posterior elements needs to be sufficient in order to avoid strong coupling to each other. Strong coupling to each other will result in poor isolation and therefore poor combined image quality. Keeping the two elements far enough apart so that the sensitivity at the middle region becomes weak enough to maintain acceptable isolation will lead to poor penetration in combined image.
Open MRI scanners where the B0 direction is vertical are especially good for shoulder imaging due to the following reasons:                (1) Large size patients who cannot fit into the small-bore horizontal scanners are more easily accommodated in vertical open MRI scanners.        (2) People who suffer from claustrophobia find more comfort in open MRI scanners.        (3) Open MRI scanners have an advantage in scanning large athletic males, typically evaluated for sports related injury. The shoulder under study can be positioned at iso-center of the scanner where the images have the best quality. Unfortunately, the shoulder coils described in U.S. Pat. Nos. 5,343,862 and 5,351,688 will not function in open MRI scanners because of the direction of the magnetic axes of the disclosed coils.        
U.S. Patent No. Des. 350,825 and U.S. Pat. No. 5,143,068 use linear loop coils. The linear coil can have good signal at center of the loop, but the coverage, uniformity and penetration are too poor for adequate clinical application, even at the humeral head, the center of region of interest for shoulder imaging.